The invention relates to implantable bone prostheses, and more particularly to knee joint prostheses.
Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone.
Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee prostheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing member. The femoral component generally includes a pair of laterally spaced apart condylar portions, the inferior or distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing component.
In a properly functioning artificial knee joint, the condylar portions of the femoral component must slide and roll freely over the articulation surface formed by the condylar elements of the tibial bearing member. Natural friction within a replaced, artificial joint can lead to the development of wear debris in which minute particles of debris (e.g., metal or plastic from the prosthesis) become dislodged and migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem thin can inhibit the proper mechanical functioning of the joint. Moreover, wear debris can lead to osteolysis and bone deterioration. When wear debris develops within an artificial joint, surgical removal of the debris or subsequent replacement of the artificial joint is often necessary.
During normal usage of a properly implanted prosthetic knee joint, load and stress are placed on the tibial bearing member. The tibial bearing member is typically made of an ultrahigh molecular weight polyethylene (UHMWPE), and friction, continuous cycling and stress can cause some erosion and/or fracture of the tibial bearing member, thus leading to wear debris. The risk of wear debris can be even greater during malalignment of an artificial knee joint, which can result from normal usage or from imperfect and/or inaccurate implantation of the prosthesis within a patient. During malalignment the load upon the tibial bearing member is not evenly distributed. Instead, excess load is placed on certain areas of the tibial bearing member. This uneven distribution of load (or edge loading) can accelerate the development of wear debris. Contact stresses on the tibial bearing member increase substantially with malalignment of the joint, thus increasing the risk that wear debris will develop when a prosthetic knee joint is subjected to malalignment conditions.
Joint replacement surgery obviously requires a tremendous degree of precision to ensure that prosthetic components are properly sized, implanted, and aligned. Imperfect sizing, implantation and alignment can lead to inadequate performance of the knee joint as well as to the presence of high contact stresses in certain areas of the prosthesis, thus leading to the possible development of wear debris.
The anatomy of patients who undergo knee arthroplasty is widely variable and can lead to difficulty in matching the standard sized prosthetic components that form a prosthetic joint. Many prosthetic components are manufactured such that similarly sized components must be used together and implanted within a patient when replacing a natural joint. That is, the femoral component, tibial bearing member, and tibial plateau that form the artificial knee joint must normally be of a matched size. If the components are not size-matched, inappropriate edge loading may develop and accelerate wear.
There is thus a need for knee joint prostheses that have a reduced tendency to develop wear debris due to the maintenance of good contact area and low contact stress between femoral and tibial components, even during the dynamics of daily activity and in various conditions of malalignment, with the options of matched or mismatched condylar sizes.
Accordingly, it is an object of the present invention to provide knee joint prostheses with improved performance and a longer useful life. It is also an object of the invention to provide knee joint prostheses having a reduced tendency to develop wear debris. A further object of the invention is to provide knee joint prostheses which are able to maintain good contact area and low contact stress between femoral and tibial components throughout normal usage conditions and in conditions of malalignment. Another object of the invention is to provide knee joint prostheses that enable the mixing of component sizes while still maintaining low contact stresses between femoral and tibial components. These and other objects will be apparent from the description that follows.